Many applications exist for battery-powered, digital wireless transmitters, primarily in cellular communications systems such as those operating under the International Telecommunication Union's Wideband Code Division Multiple Access (WCDMA) standard or other so-called “3G” standards. Such transmitters use one or more amplifiers to amplify components of the input signal to be transmitted. These components are amplitude and phase components in the case of a polar transmitter and in-phase and quadrature components in the case of a Cartesian transmitter.
A highly linear amplifier distorts the signal the least and so is most favored from a standpoint of signal quality. Unfortunately, highly linear amplifiers use relatively large amounts of power and numbers of highly accurate components, making them relatively power consumptive, large and expensive. Though they perform the best, they are thus disfavored in many wireless applications, particularly those that require low-cost transmitters. The amplifier that is best suited overall for low-cost, battery-powered wireless transmitters is a simple amplifier having significant nonlinearity. See, for example, FIG. 1A, in which a nonlinear amplifier 110 distorts a substantially sinusoidal input signal.
Predistortion is often used to compensate for this nonlinearity, resulting in a linearization of the output of the amplifier. The theory underlying predistortion is that, if an amplifier's distortion characteristics are known in advance, an inverse function can be applied to an input signal to predistort it before it is provided to the amplifier. Though the amplifier then distorts the signal as it amplifies it, the predistortion and the amplifier distortion essentially cancel one another, resulting in an amplified, output signal having substantially reduced distortion. See, for example, FIG. 1B, in which a digital predistorter 120 predistorts the substantially sinusoidal input signal such that the output signal is likewise sinusoidal.
In digital transmitters, digital predistortion (DPD) is most often carried out using an LUT that associates output values with input signal values. Entries in the LUT are addressed using samples of the input signal. The output values retrieved from the LUT are used either to predistort the samples (an “inverse gain” configuration) or in lieu of the samples (a “direct mapping” configuration). In modern applications such as WCDMA, samples are transmitted at a very high rate. Thus, the predistorter needs to be able to look up and retrieve output values very quickly.
The spacing between successive LUT entries defines the mapping accuracy. LUTs are capable of providing a nonlinear mapping for signals with a large dynamic range, which is essential for complying with 3G standards such as WCDMA. Most applications employ a uniformly spaced LUT. However, the performance of a uniformly spaced LUT degrades considerably at lower input signal levels because an increase in the signal dynamic range relative to the quantization level substantially decreases the signal-to-noise ratio (SNR). Nonlinear spacing techniques in which more entries are heuristically placed where the mapping is nonlinear can, in theory, mitigate this problem. Unfortunately, although these techniques improve LUT performance in some applications, they do not handle arbitrary nonlinearity well and do not work well in demanding applications, such as transmitter linearization.
A predistortion LUT is typically created when a transmitter is calibrated at the factory. Unfortunately, a factory-calibrated predistortion LUT often fails to linearize the amplifier adequately under varying operational conditions (e.g., temperature, voltage, frequency and voltage standing-wave ratio, or VSWR). Aging, especially in WCDMA and other 3G transmitters, only exacerbates the inadequacy.